Plenum boxes comprising pressure distribution pipes in which the pressure distribution pipe is configured to receive an incoming air flow from a connected ventilation system are known. It is also known that the pressure distribution pipe may comprise a regulating damper which can be regulated in such a way that the resulting flow of air into the plenum box is linear. Prior art uses rotary dampers in pressure distribution pipes. To achieve low noise values, such dampers are traditionally perforated. A problem with such dampers is that they do not entirely shut off an air flow in the closed position, with the result that there is always a certain flow of air into the plenum box. A further problem of prior art is being able to effect quick and sudden adjustments of a flow in the ventilation system, e.g. if a sudden pressure drop in the system occurs or is desired to occur. In traditional plenum box technology it is usual for there to be in a plenum box a fixed rotary damper made of, for example, perforated sheetmetal with, for example, a 50% aperture cross-section, disposed inside the plenum box above the outlet. This has been necessary in order to achieve uniform spread and distribution of the air flow downwards in the delivery facility, resulting in an unoptimised spread of air flow into the room. A disadvantage which has occurred with this fixed position of a rotary damper above the outlet is that it has made it difficult, and in many cases even impossible, to clean inside the pressure distribution pipe easily when the plenum box is opened.